Preheat oven to 350 degrees F. Lightly grease a 8x8 inch pan.
In a large bowl, mix together flour, sugar, brown sugar, baking powder, baking soda, and salt. Set aside.
In a small mixing bowl, mix flour, brown sugar and cinnamon. Add the melted butter and mix until crumbled.
In a medium bowl, mash the banana and mix in the sour cream, oil, egg, and vanilla. Add to the flour mixture and stir until combined.
Spread half the batter evenly into the prepared pan. Sprinkle with 1/3 of the streusel. Pour the remaining batter evenly over the struesel. Top with remaining struesel and walnuts.
Bake for 30-35 minutes until toothpick comes out clean.